Eruption is defined as the continuous tooth movement from the stage of the dental bud to the occlusal contact.
In our life time we get 2 sets of teeth the Primary dentition and the permanent dentition. First the primary dentition are seen, which on dislodgement give space for the permanent teeth to erupt.

In teh image we can see the development of the permanent dentition in the presence of the decidious teethEruption
The tooth starts its passage through the alveolar crust when two thirds of the root is formed, and the emergence of the tooth through the alveolar gingiva when three fourths of the root is completed.